Why Coreelec, Alexelec have Python 3? Only you cannot use Python 3? why?

Installing both Python3 (that cannot be used by Kodi) side-by-side with Python2 (which is used by Kodi) makes no sense for a size optimised distribution like LibreELEC that targets Kodi - including Python3 at this time would benefit almost nobody and be a waste of space for everyone else. LibreELEC will replace Python2 with Python3 during the Kodi 19 development cycle.

You know the use case could be to make the transition easier. And since hyperion is using it so you could update it.

Until Kodi 19 uses Python 3 as a language interpreter, including Python 3 (as an add-on or not) would not help with the transition as Kodi would continue to use only Python 2 when executing Python add-ons.

Installing Python 3 into the image as well as Python 2 is a very niche requirement, which is why it hasn't been done so far. And for much the same reason nobody has submitted a Python 3 add-on as there isn't really the demand for it.

Also, the version of hyperion that is supported by LibreELEC 9.0 is using Python2. If someone wants to use a different and unsupported version of hyperion that has different Python language requirements then that is not our problem to solve.

@Crylogic hasn't explained what his use case actually is - he only asked why we don't include Python 3, which has now been explained.

And to my knowledge a package for a Python 3 add-on does not exist, but if anyone wants to submit it as a PR we can consider it, or better yet a community builder can build it/host it as it will have a very short life span.